Extrusion International 6-2022

48 Extrusion International 6/2022 RECYCLING foreign substances, but also with a large number of different polymers. Figure 1 shows that ABS, PS and PP, for example, make up the majority of plastic materials. The other plas- tics described above account for less than half. It therefore makes sense to con- centrate on the significant amounts of plastics during reprocessing. Re- covery of plastics that are present in the low single-digit percentages is unlikely to be worthwhile. Processing technologies The “bulk plastics” in electronic scrap, for example, are PS, ABS and PP. These materials lie fortunately in density ranges that are accessible for the use separation processes based on density, such as the float- sink separation technique. This is a particularly economical process, which - assuming fractions are of appropriate density – results in con- centrated plastic that can then be further processed by dry electro- static processes. For example, it is easy to separate polyolefins from a plastic mixture us - ing a simple water separation stage. PS and ABS can be separated using water with a higher density (e.g. a salt solution). A salt solution in water then enables production of highly concen- trated PP/PE and PS/ABS mixtures. In particular, undesirable flame- retardant plastics go through this process into the heavy residual frac- tion, where PVC, glass, residual met- als and other undesirable substances also end up. This residual fraction is thermally recycled, for example. Processing of PS-ABS mixtures PS and ABS are the "favourites" in terms of added value and yield - but only if these materials are available in high purity. ABS and PS in a non- separated mixed fraction can only be processed with great difficulty with the aid of expensive additives. While optical sorting methods using infrared spectroscopy, or other meth- ods, are still in the starting blocks due to the predominantly black colour of plastic mixtures, this type of sepa- ration has not been a problem for electrostatic separation technology, already for decades. Electrostatic sep- arators can easily separate black ABS- PS mixtures into pure fractions. Electrostatic plastic-from- plastic separation ABS and PS essentially differ in their electrostatic chargeability. When one of these two plastics in a mixture of the two is selectively charged in an electrostatic sepa- rator, the ABS becomes positively charged and the PS negatively, due to different “dielectric constants”. As the charging is highly selective, subsequent separation in a high- voltage field results in very good pu- rity of both products. The prerequisite for your suc- cessful separation is that the ABS- PS mixture has the correct particle size of < 10 mm and is dust-free. The most important criterion, how- ever, is the moisture content of the material. Only dry ground material can be sufficiently electrostatically charged. Electrostatic separators in practice Electrostatic separators for plastic- from-plastic separation are supplied as complete systems including a charging unit and high-voltage electrode system for material separation. For technolog- Picture 2: Clean PS fraction Picture 3: Fraction clean ABS Picture 4: hamos EKS - Plastics / plastics separator Picture 5: hamos KRS - Processing of WEEE plastics Picture 1: Mixed plastics